1. Field of the Invention
This invention relates to a video signal reproducing apparatus and more particularly to an apparatus which reproduces, by means of two heads, a video signal from many evenly spaced recording tracks formed on a recording medium.
2. Description of the Prior Art
In the following description, magnetic recording/reproducing apparatus of the rotary two-head helical scanning type (hereinafter called VTR), to which the present invention is advantageously applicable, will be discussed by way of example.
A tendency toward density recording performed with high density on a magnetic tape with a VTR has advanced during recent years. Accordingly, the width of recording tracks has in general been reduced. As a result, the necessity for automatic tracking control hs increased for accurately determining the position of a magnetic head relative to a magnetic tape. It is also necessary that a magnetic head accurately scan and reproduce a record from the recording tracks on the magnetic tape during reproduction. A method for automatic tracking, which has recently become popular, is as follows: A pilot signal is recorded in each recording track and is superimposed on a video signal recorded there. In reproducing the record, the pilot signal levels reproduced from adjacent recording tracks loacted behind and in front of a recording track being traced for reproduction are compared, thereby forming a tracking signal. Then, the magnetic tape travel is controlled based on this signal.
Meanwhile, there have been proposed and practicalized VTR's of a different kind having two rotating heads, which are called unbalanced type heads. These two rotating heads have different head widths for reducing a noise bar produced during video signal reproduction at a varied speed for still picture reproduction, slow motion picture reproduction, or the like. The VTR using such unblanced heads must have overlapped recording tracks for uniform recording track width at the time of recording. Therefore, in VTR's of this kind, the two heads secured to a rotary cylinder must have their end surfaces on one side aligned with each other in their travelling direction. If the above automatic tracking method is applied to a VTR using such unbalanced heads, control over the magnetic tape travel causes the center line of the recording tracks to be traced by the centers of the heads. However, in VTR's of this knid, the operation of a control system becomes unstable with the magnetic tape travel controlled in such a manner as to have the centers of both the heads trace the center line of the recording track because, as mentioned above, the two heads have their end surfaces on one side aligned in their travelling direction this means that their center lines are not aligned with each other. In other words, the rotating surfaces at the centers of these two heads deviate from each other in the direction of the axis of rotation. Therefore, the controlled object of the magnetic tape travel control changes every time one reproducing head is switched over to another. Therefore, a control signal varies at the time head switch-over if control is performed in such a way as to have the center line of the recording track traced by the centers of both of the heads. This results in an unstable control system. Furthermore, even when the rotating surface at the center of one head is the same as that of the other, the difference in head width varies the control signal of the above tracking method at the switch-over time from one head to the other and, thus, renders the control system unstable.
It is possible to slove this problem by processing the above tracking signal through a low-pass filter of a low time constant or through a comb-shaped filter, which removes a frequency component related to the period of head switch-over before the signal is used for control of the magnetic tape travel. However, if the rotating surfaces of the centers of the two heads greatly deviate from the direction of the axis rotation, the two heads and particularly the one having a narrower head width would not track satisfactory.
It is therefore a general object of the present invention to eliminate the shortcomings of the prior art apparatus mentioned in the foregoing.
It is a more specific object of the invention to provide a video signal reproducing apparatus which eliminates the unstableness of a control system for the travel of a recording medium attributable to the difference in head width between the two heads.
It is another object of the invention to provide a video signal reproducing apparatus which eliminates the unstableness of a recording medium travel control system attributable to deviation of the rotating surfaces at the centers of the two heads in the direction of the axis of rotation.
It is a further object of the invention to provide a video signal reproducing apparatus which is capable of controlling the movement of a recording medium to ensure that the two heads always perform satisfactory reproduction.